Service tax liability

sir/ma'am,

we are into slitting of CRNO/CRCA steel coils , the material being sent by our customers.

we do slitting jobwork for them.and charge our labourcharges for the same.

all our customers are under excise.are we liable to pay service tax.

and there are few customers who are not under excise.and to such customers is there any limit .

and we are registered under SSI.

please clarify.

regards,

KAMAL.K